1. Field of the Invention
The present invention relates to an image synthesizing method using a plurality of radar reflection waves and an image radar apparatus for an aircraft using the method and in particular, to a multiple-radar wave image synthesizing method for catching a front information in the aircraft flying direction, and an image radar apparatus using the method for an aircraft.
2. Description of the Related Art
Conventionally, there have been developed various types of aircraft radar apparatuses for various purposes. All of these radar apparatuses have a common target how to improve their resolution levels.
As a method to increase the resolution level in an aircraft radar apparatus, the pulse Doppler method has been used. This pulse Doppler method is characterized in a high resolution in the distance direction.
Moreover, as a high-resolution radar apparatus to be mounted on an aircraft, there has been known an Synthetic Aperture Radar or SAR.
Furthermore, there is known a phased array type radar having a plurality of arrayed antennas as a small size radar apparatus.
In this phased array type radar has an antenna block as an independent small unit so as to change the direction of the antenna rapidly using the signal phase synthesis technique.
Moreover, the aforementioned phased array type radar has been further developed into a conformal antenna in which an aircraft wing member is made together with an antenna. In this radar apparatus, there is no problem of the antenna aperture length. It has been confirmed that the spatial resolution is sufficiently improved.
On the other hand, there has been developed a sun-radio wave heliograph has been developed as a category near to the phased array type radar in which an image is created by radio information phase synthesis. (For example, see Nakazima et al "The Nobeyama Radioheliograph-Hardware System--(PP. 19-31)".
As is clear from the aforementioned document, in the sun radio wave heliograph, radio waves generated by the sun is received a plurality of antennas arranged in T-shaped formation and two-dimensional phase synthesis is simultaneously performed so that the sun image can be displayed with resolution of 10 seconds (degree).
However, each of the conventional radar apparatuses has a problem. Firstly, in the case of the pulse Doppler method, it is possible to obtain a high resolution in the distance direction but the resolution in the direction orthogonal to the distance direction is determined by an antenna beam width and cannot be increased than this.
That is, a pulse wave produced from a transmission antenna is reflected by an object and a reception antenna receives composite reflections from objects at an identical distance. In this case, if a single reception antenna is used, the antenna beam B spread is determined by the antenna configuration and size (aperture length). Accordingly, as shown in FIG. 9, it is impossible to separate a plurality of objects, for example, two targets K.sub.1 and K.sub.2 at an identical distance.
As for the forward distance direction, there is no case that there are two objects at an identical distance if on the plane land. However, if there is a mountain, as shown in FIG. 10, there may be two objects K.sub.3 and K.sub.4 at an identical distance and cannot be isolated from each other.
On the other hand, Synthetic Aperture Radar (SAR) which is a high-resolution image radar uses the Doppler effect accompanying the relative movement between the aircraft and an object on the earth. Accordingly, the front and the back of the aircraft cannot be seen because there is no Doppler effect.
Furthermore, in the phased array type radar, reception signals of a plurality of arrayed antennas are synthesized and the arrayed antennas constitute an independent small unit. Accordingly, the aperture length of the entire antenna unit has a small aperture length, which results in a low resolution.
Moreover, in a radar apparatus using a conformal antenna, there is no aperture length problem but it takes much time for a data creation for one screen because beam scan is performed by changing phase synthesis parameters. Accordingly, this type of apparatus is not useful for a successive front monitoring.
As for the successive monitoring the front of an aircraft with a sufficiently high resolution, an antenna having a real large aperture is used such as AWACS produced by Boeing Co., Ltd.
However, the technique to use an antenna having a real large aperture has a problem that the air resistance is large and the weight is also increased, which are disadvantages for an air craft.
The aforementioned sun radio wave heliograph which is a radio information imaging apparatus does not transmit an electric wave and installed at a fixed point, which is a disadvantage for an aircraft.